belong to a group of hard, vitreous minerals that are composed of silicates
of magnesium, calcium or iron. Alternatively they can be composed of manganese
with aluminum or iron.
They vary across a wide color spectrum, with their dominant color determined by mineral composition.
Clear red garnets are valued as jewelry. In addition, they are useful as industrial abrasives.
Six-ray star garnets can be found in only two places in the world - Idaho and India. Seeming to float just below the surface of the stones, the stars appear magically when the stone is held in sunlight. These stars (the more valuable garnets have stars with six arms, but some garnets have stars with four arms) are caused by a residue of titanium dioxide trapped in the garnet crystal. The result is so beautiful that this stone has been named Idaho's state gem.
Raw garnets - both intact twelve-sided crystals up to two inches in diameter and broken pieces - are found in streambeds in the Emerald Creek area near Fernwood. The gemstones are not on the surface, but mixed with the sand, rocks, roots, and dirt between the stream and the bedrock several feet below. Separating the gems from the surrounding soil is a difficult and dirty job, but one that draws thousands of visitors every summer to the digging area managed by the Forest Service.
Lead is a heavy,
bluish-gray metallic element with atomic symbol Pb. The metal is relatively
soft and malleable. It typically occurs in nature as sulfides (such as Galena),
and is often associated with silver and zinc.
These deposits are found in the Silver Valley near Kellogg along the Coeur d'Alene River in north Idaho and near Hailey and Bellevue in south central Idaho. These generally have supported the mining economy of Idaho over the last 100 years, but their importance is decreasing. These veins are formed when sulfide minerals fill void spaces in cracks or shear zones.
Coeur d'Alene District:
The Coeur d'Alene district in northern Idaho is one of the major lead-zinc-silver producing areas of the world. Since mining began in the early 1880s, mines in the 300-square-mile district have produced more than 2.89 billion dollars worth of silver, lead, zinc, copper and gold.
The country rock (host rock) consists of six formations of the fine-grained, siliceous, Precambrian Belt Supergroup. The sediments are intruded by several types of small stocks and dikes. The structural geology of the area is complicated by a variety of folds and faults of diverse acres and movements. The district is at the intersection of the west-northwest-trending Osburn fault and a north-trending anticlinal uplift.
Six periods of mineralization ranging in age from Precambrian to Tertiary have been identified. The main period of mineralization probably occurred during the Late Cretaceous.
The productive veins, apparently controlled by deep fractures, trend northwesterly. Although many veins crop out at the surface, some apex several thousand feet below the surface. Depth appears to have had little effect on the occurrence or type of ore.
is a silver-white metallic element with atomic symbol Mo. Although the most
important use of molybdenum is as an alloy to enhance the strength and durability
of steel, it has many other uses.
Commercially useful molybdenum is found in one location in Idaho. The Thompson Creek molybdenum mine, owned by Cyprus Mining Company, between Challis and Salmon is located in altered Cretaceous granitic rock. The mine is an open pit and produces from a stockwork of tiny veins on the edge of a granitic body dated at about 87 Ma.
Exploration of the mine began in 1967 and continued for 14 years. In 1978 the decision was made to develop the property. The ore body consists of an igneous granitic stock of Late Cretaceous age intruded argillites of Mississippian age. The intrusive and sedimentary rocks are overlain by Challis volcanics of Eocene age. The molybdenite occurs primarily veins and veinlets disseminated throughout the deposit.
The ore body has an estimated 181 million tons of reserves, averaging 0.18 percent molybdenite. Actual mining and ore processing began in 1983, more than 17 years after Cyprus staked its claims. This mine, with associated facilities and equipment, is one of the most modern large open pit mines in the western United States.
is a solid, nonmetallic element with atomic symbol P. It can exist in two
forms - one yellow, poisonous and highly flammable, the other is red, less
poisonous and less flammable. An interesting property is that water does not
put phosphorus fires out, the element will continue to burn under water. Phosphorus
is an extremely useful element and is of great commercial importance. Production
of the element and byproducts (such as fertilizer) contributes significantly
to Idaho's economy.
The southeast Idaho phosphate industry produces the greatest amount of income to the state of any precious mineral. The deposits are found in the Permian Phosphoria Formation and are mined by open pit methods. These are stratigraphically controlled concentrations of P205. Concentrations of about 15 to 30% are minable.
Southeastern Idaho contains both the thickest and richest phosphate deposits in the western United States. Idaho phosphate accounts for as much as 14 percent of the total phosphate produced in the United States, and is second only to phosphate production in Florida and North Carolina. Phosphate is of economic interest primarily for production of fertilizer and a myriad of products from detergents to soft drinks.
The geology of southeastern Idaho was first studied by members of the 1877 Hayden Survey, They recognized some of the broad structural features and the Carboniferous and Triassic rocks in the area however, the Permian phosphate deposits were not discovered until 1889 by Albert Richter. Since that time
|Click here to look at a distribution map of the Phosphoria Formation.|
geologic efforts have
focused on the area's vast phosphate resources and secondarily as a possible
source of vanadium and uranium.
The Idaho phosphate deposits are sedimentary rocks that occur in the Permian Phosphoria Formation. The Phosphoria Formation is centered in Idaho, but extends regionally into northeastern Nevada, northern Utah, western Wyoming, and southwestern Montana.
In southeastern Idaho, the Phosphoria is subdivided into three members, in ascending order: the Meade Peak Phosphatic Shale Member, the Rex Chert Member, and the cherty shale member.
The rich phosphate beds occur in the Meade Peak Phosphatic Shale which reaches a maximum thickness of about 230 feet in southeastern Idaho. Since the Meade Peak is a relatively soft lithologic unit, it is rarely exposed, but is recognized at the surface by the presence of adjacent erosion resistant units. The region is structurally complex and strongly folded so the bedding of the Meade Peak is commonly tilted to steep angles. Because of differential erosion and the tilting, the Meade Peak is characterized by a dominant surface swale that occurs parallel to the strike of the bedding.
Phosphate occurs in the Meade Peak in sedimentary rocks called phosphorite and phosphatic mudstone.
Phosphorites are composed dominantly of phosphate minerals, varieties of the mineral apatite, that occur in tiny spherical particles called ooliths and peloids (less than 2 mm in diameter) and in larger nodules and fossil fragments. Ooliths, the most abundant of the smaller particles, are accretionary particles composed of concentric bands around a nucleus. Individual phosphatic particles are hard and nearly black. Weathering of phosphate-rich rocks produces a bluish-white coating referred to as "phosphate bloom" which aids in recognition of phosphate in surface samples.
Deposition in a Shallow Sea:
The Phosphoria Formation was deposited in a shallow sea during the Permian Period. While phosphate-rich water may have originated and upwelled from deeper ocean waters, phosphate-rich sediment accumulated, was reworked and deposited as sediment in the relatively shallow waters of an epicontinental seaway or embayment. Some phosphate deposition is also thought to have occurred as a result of diagenetic processes, that is, formed by precipitation of apatite within intergranular spaces and pore water after initial deposition of sediments. The formation and concentration of spherical peloids and oolites, as described above, required agitation by wave action and winnowing of sediment that would occur in a shallow-ocean environment in times of maximum regression of the sea.
The geometry of the southeastern Idaho phosphate deposits is attributable to the complex structural geologic setting of the region. Most of the structural deformation of the region is the result of major episodes of tectonic activity occurring in the Mesozoic and Cenozoic eras. Southeastern Idaho is part of the Idaho-Wyoming thrust belt where regional compression from Late Jurassic to Cretaceous time resulted in substantial crustal shortening. This crustal shortening, achieved by thrust faulting and folding, is responsible for the effective doubling of the overall stratigraphic thickness. In Tertiary time, regional extension of the Basin-and-Range Province produced north- to northwest-trending normal faulting in southeastern Idaho. Recent seismic activity indicates that Basin-and-Range-type deformation continues to the present. As a result of the overprinting of these structural events, the Phosphoria Formation and associated stratigraphic units are folded, faulted, and tilted.
Mining of Phosphate:
In southeastern Idaho, phosphate is mined in large open-pit mines. Because the phosphate beds are tilted and oriented along large folds, open-pit mines in southeastern Idaho are long, linear trenches excavated along strike of the bedding. Phosphate rock is mined downward along the dip of bedding to an economic depth (currently as much as 300 feet or more). The economic viability of a particular phosphate deposit is related to many factors including the thickness and grade (P2O5 content) of the rock, the amount of overburden that needs to be stripped off to reach the phosphate beds, location, market considerations, and structural geologic complications. The depth to which the phosphate is weathered is also a critical factor. The more weathered the phosphate is generally the richer the ore and the more easily mined. Phosphate ore is mined using large mine shovels and scrapers and is transported by ore truck, rail line, or slurry pipeline to processing plants in Soda Springs, Pocatello, and other locations in the region.
Processing of Phosphate Ore:
Phosphate rock in southeastern Idaho is either: 1) processed into chemical fertilizer products by a wet process that dissolves phosphate rock with sulfuric acid to produce phosphoric acid; or 2) processed into elemental phosphorus by smelting a mixture of agglomerated phosphate rock, silica, and fine-free coke in a submerged-arc electric furnace. Phosphate rock in the western phosphate field is generally classified as followed:
* High grade (or acid grade) is plus 31% P2O5
* Medium grade (or furnace grade) is 24 to 31% P205
* Low grade (or beneficiation grade) is between 16 and 24%P2O5
High-grade rock is used directly in fertilizer plants- medium-grade rock can be used directly in the elemental plants; and low-grade rock needs to be upgraded (beneficiated) to furnace or acid grade.
Potential Byproducts of Phosphate Production:
Phosphatic units of the Phosphoria Formation are enriched in many rare elements; however, only vanadium is currently being recovered as a byproduct of elemental phosphate production in the western phosphate field. Vanadium is used as an alloying element in steel to improve its strength, toughness, and ductility. Other elements with potential as byproducts are uranium, fluorine, rare earth elements, silver, cadmium, chromium, molybdenum, arsenic, selenium, strontium, tellurium, and zinc. Some elements such as fluorine, uranium and its decay products, cadmium, thallium, and mercury should be recovered to avoid environmental risks from phosphate products or waste materials from phosphate processing.
Reclamation of Mined Lands:
While open-pit mining of phosphate has definite impacts on the natural environment, substantial progress has been made in southeastern Idaho to mitigate impacts and reclaim surface disturbance through the cooperative efforts of phosphate companies with Federal and state agencies. Land reclamation is the process of returning land disturbed by mining to productive uses. Specifics of the mine operation dictate the type and timing of the reclamation done. Reclamation of the surface disturbed by mining includes the regrading of waste dumps and mine cuts to stable gradients and the establishment of vegetation. The establishment of the suitable vegetative cover aids in improving the soil, protecting the land surface from erosion, improving aesthetic values, and in restoring the land to productive uses such as grazing and production of crops.
Future of Idaho Phosphate:
As in the past, the future of the Idaho phosphate industry will depend on the market demands for phosphate fertilizer and chemical products which is strongly influenced by fluctuations in the agriculture and consumer industries.